For example, Japanese Patent Application Publication No. 2005-89100 discloses an image forming apparatus including a sheet processing apparatus. The image forming apparatus includes a saddle stitching apparatus such as a stapler for sequentially fetching sheets with an image formed by the main body of the image forming apparatus and then stitching the neighborhood of the center of the sheets and a sheet processing apparatus for folding the stitched sheet bundle.
Japanese Patent Application Publication No. 2005-89100 discloses a sheet processing apparatus including a projected plate to project the central part of a sheet bundle and permit it to be held by a pair of rollers and a pair of rollers for folding the held sheet bundle in two.
The sheet bundle which is discharged from the main body of the image forming apparatus and is subjected to the saddle stitching process at the central part by the saddle stitching apparatus is conveyed into the sheet processing apparatus. Here, the sheet bundle is conveyed between the folding roller pair and the projected plate and is arranged so that the central part (the bound part) of the sheet bundle comes to the position facing the projected plate.
The paired folding rollers are driven to rotate in a predetermined direction and hold and crease the sheet bundle, so that a nip portion having a nip pressure due to pressurizing the contact portion of both rollers is formed.
The projected plate is controlled so as to make a reciprocating motion of moving from the start position to the operation position where the end thereof approaches the nip portion and then returning to its original start position.
The sheet bundle the central part of which is pressed by the projected plate makes contact with the paired folding rollers and is conveyed to the neighborhood of the nip portion by the friction with the contact surface. And, the sheet bundle is folded in two by the nip portion and is subject to the folding process. The sheet bundle which is folded and bound is sequentially discharged onto the bundled sheets receiving tray for receiving sheet bundles.
However, the constitution of Japanese Patent Application Publication No. 2005-89100 aforementioned is effective in a sheet bundle subjected to the saddle stitching process, though in a sheet bundle not subjected to the saddle stitching process, the sheets are not fixed mutually, so that sheets subjected to the folding process in the state that the ends thereof are not trued up may be generated.
For example, a sheet bundle composed of three sheets, in the situation before held by the paired folding rollers, is composed of a first sheet, a second sheet, and a third sheet from the side opposite to the paired folding rollers. Namely, the third sheet is opposite to the end of the projected plate and the saddle stitching process is not performed.
The sheet bundle pressed by the projected plate makes contact with the paired folding rollers and is conveyed to the neighborhood of the nip portion by the friction with the contact surface. In this period, there is no connection such as a binding process mark on the surface of the third sheet in direct contact with the projected plate and the frictional force is reduced due to a small contact surface between the projected plate and the third sheet, so that the third sheet slides down by its own weight, and the end of the projected plate is shifted and pressed from the central part of the third sheet and is held by the nip portion. By doing this, the end portion of the sheet bundle is shifted.
The adhesion between the first, second, and third sheets of the sheet bundle, since the saddle stitching process is not performed, depends on the friction with the contact surface of each sheet. When conveying the sheet bundle pressed to the neighborhood of the nip portion by the projected plate to the nip portion, the rotational power of the paired folding rollers firstly conveys the first sheet in direct contact with the paired folding rollers due to the friction with the contact surface. And, the second sheet and third sheet are similarly conveyed due to the friction with the contact surface with the first sheet conveyed. Here, the friction between the respective sheets is not ensured sufficiently, so that the second sheet is delayed from the first sheet and enters the nip portion. By doing this, the end portion of the sheet bundle is shifted.
As mentioned above, a problem arises that the end portion of the sheet bundle after the folding process becomes uneven, and the alignment is impaired, and defective binding is caused.
Further, as the surface of surface coated paper becomes smoother, the friction coefficient is reduced, so that the frictional force is lowered. Further, as the contact surface between the respective sheets is reduced, the surface for realizing the frictional force is not ensured, so that the frictional force is lowered.